The molecular events involved in RNA tumor virus gene expression, and the control of gene expression, will be studied in great detail. Synthesis, processing, transport, utilization, and breakdown of specific viral RNA species representing individual viral genes will be examined utilizing cDNA probes which are specific for the env and src genes (coding for the envelope glycoprotein and sarcomagenic function, respectively) and with three other specific cDNA probes representing, collectively, the entire viral genome. A number of low molecular weight (21-31S) viral specific RNAs have been identified in both normal and infected cells. These contain env and src genetic information, but do not contain viral sequences located at the 5' end of the viral genome. Experiments will be performed to identify the function of these smaller RNAs, and to translate them in cell free protein synthesizing systems. Particular emphasis will be placed on the RNA molecules containing src sequences, since these may be the active messengers for the transforming gene. The process of virus recombination will also be studied, and experiments are described which should identify the step at which the recombination event takes place.